1. Field of the Invention
The present invention relates to a polling type data communication system for data communication between a master station and a plurality of slave stations which are connected by a dedicated line used in a multidrop system.
In more detail, the present invention relates to a data communication system which permits coexistence of digital lines and analog lines and utilizes polling type data communication, where a master station and a plurality of slave stations are connected by digital lines or analog modem lines in hierarchical form.
2. Description of the Prior Art
Recently, a variety of analog and digital transmission equipment has been developed for data communication. A system connecting the analog lines and digital lines allowing coexistence of analog and digital equipment was needed, and therefore has been established. An example of such a system is shown in FIG. 1 in which items 111-119 indicate a variety of transmission equipment. A solid line indicates an analog transmission line while dotted line indicates a digital transmission line. For monitoring such a system, the transmission equipment of the highest level of the hierarchy includes a monitoring master station, while the multiplicity of transmission equipment includes monitoring slave stations #01-05. Slave station #01 includes repeating equipment to allow coexistence of the analog and digital lines to which it is connected. The monitoring master station collects data with the auxiliary line voice channel of the transmission line used as the dedicated line. U.S. Pat. No. 4,727,541 illustrates a typical transmission line having auxiliary and main channels. In this case, the master station gathers information in the polling system for the lower level slave stations.
As explained above, gathering information through this system permitting coexistence of digital lines and analog lines effects communication through repeating equipment provided between such analog line and digital line. A conventional embodiment of such repeating equipment as incorporated into slave station #01 is shown in FIG. 2. Reference numeral 11 denotes a modem circuit which demodulates the analog signal to be input through the analog line or modulates the signal reversely transmitted from the stations of lower level; LSI 12 is communication interface circuitry to convert the serial input signal into a parallel signal; microprocessor 13 stores the converted input signal into buffer 14 for adjustment of output timing; LSI 15 is communication interface circuitry to convert the parallel input signal into a serial signal; driver/receiver 16 sends the signal to the lower level stations or receives the signal from the lower level stations.
The process by which a digital signal is input from the lower level slave stations is as follows: When a digital signal is input from the lower level slave stations, driver/receiver 16 receives this signal and is then transferred to microprocessor 13 through LSI 15. Microprocessor 13 accumulates the input signal in buffer 14 until the carrier signal which is to be sent to transmit the analog signal to the line in the side of stations of higher level by the modem circuit 11 is generated and stabilized. An oscillator in modem circuit 11 is always in the oscillating condition, however the oscillation output is not constantly transmitted to the line extending toward the higher level stations. This is because if branched lines simultaneously generate a carrier signal, this conventional system cannot discriminate between various higher level stations due to the method of processing information employed by the polling system. Therefore, the carrier is first transmitted to the line toward the higher level stations after the signal responsive to the polling is received from the lower level stations providing the condition to enable signal output. Since a reasonable time is required until the carrier signal is stabilized, the microprocessor has a buffer to output the signal to the modem line after the carrier is stabilized.
This operation is carried out for every character of data, and therefore generates delay time in the signal transmission. Each polling process requires a longer time resulting in a deterioration of process quality. Since the signal must be transmitted to the modem 11 after the carrier is stabilized repeating equipment as explained above is required causing the additional problem of adding complexity to the circuit structure.